Wordpress - Easily Navigate Within a Section of Pages

EasilyNavigateWithinASectionOfPages.php

// Taken from: https://speckyboy.com/wordpress-snippets-secondary-navigation/
/*
This code makes for a great secondary navigation menu that could be used, for example, within a sidebar. It automatically displays the pages within the current section you’re browsing.
As an example, let’s say your site has an About Us page, with subpages for History, Mission Statement and Staff. Users will be able to more easily navigate between the various pages within this section as a bulleted list of links.
*/
<?php 
if (  $post->post_parent ) { 
    $children =  wp_list_pages( array( 
        'title_li' =>  '',
        'child_of' =>  $post->post_parent, 
        'echo'     => 0 
    )  ); 
} else { 
    $children =  wp_list_pages( array( 
        'title_li' =>  '', 
        'child_of' =>  $post->ID, 
        'echo'     => 0 
    )  );
} 
if (  $children ) : ?> 
    <ul> 
        <?php  echo $children; ?> 
    </ul> 
<?php endif; ?>